The present invention relates to the measurement of surface roughness and, more particularly, to methods and apparatus for such measurements.
An essential part of quality control in many manufacturing operations is the measurement of the quality of the surface finish of the parts produced, and, more particularly, the roughness (or smoothness) of the surface. Although appearance or aesthetics are often the basis for concern for evaluating the roughness of a surface, equally often the primary reasons relates to the specific applications for the parts. For example, bearing surfaces which are too finely finished will not be able to hold the lubricants needed to minimize wear; the microscopic pits and grooves--of the finish--serve as the reservoirs for the lubricants. Parts such as turbine blades which are subject to high stress may fail due to stress corrosion and metal fatigue if their finish is too coarse.
Manufacturing processes which involve working of a part generally result in scratches, dents and gouges. As an example, the surface finish at the root of a turbine blade may be considered to exhibit a series of fine, overlapping scratches; the finer the finish, the smaller and shallower the scratches. These scratches may be regarded as microfractures which barely penetrate the surface. However, as the blade flexes under stress, the forces transmitted to the blade root cause the scratches to enlarge and extend deeply into the metal until, eventually, the blade will snap. Since the fracture of even a single turbine blade at a high rotary velocity can cause major damage, the aircraft engine manufacturers regard each turbine blade as a potential weak link in the chain of components whose composite mean time to failure determines engine service life. Therefore, the many turbine blades which go into an engine are inspected for surface finish, and this is an extremely costly and time consuming process.
One commonly used method of inspection is visual comparison with a sample of known surface roughness. The inspector determines whether the workpiece is better or poorer than the sample standard. Comparison with a range of standards then serves to bracket the surface finish of the workpiece as lying between that of the most similar coarser finished and finer finished samples.
Another commonly used method of surface finish measurement is the profilometer. This typically consists of a fine stylus connected to a linear variable differential transformer. The workpiece is securely mounted on an inspection surface plate and the stylus, mounted on an adjustable holder, is drawn across the lay, or grain, of the workpiece. This method also typically uses sample standards of known surface finish to calibrate the profilometer. It commonly provides repeatably accurate measurement of surface finishes down to 5 microinches. With exceptional care, surface finishes as fine as 3 microinches can be measured.
Both of these methods have serious drawbacks; the visual comparison method is highly subjective, while the profilometer method is only useful for measurements across the lay of the finish. Furthermore, both are primarily useful for flat surfaces. Workpieces of complex shape, or with recessed surfaces are difficult, if not impractical, to measure by either of these methods. Therefore, there have been efforts in recent years to develop alternative methods for evaluating surface finish, which methods are not subjective and do not require physical contact with the workpiece. A further goal has been to devise a method which can be used by relatively unskilled personnel, and which lends itself to automation in order to reduce the high cost of surface finish evaluation.
There have also been efforts to develop techniques which measure and analyze light reflected from the workpiece surface, and these can be classified into the following groups based on the methodology:
1. Light projection technique PA1 2. Interferometry PA1 3. Measurement of reflected beam positional variations PA1 A=constant [intensity difference between parallel and perpendicular to the lay] PA1 .theta.=angle between incident beam and the lay of workpiece PA1 B.sub.1 =highest calibrated surface roughness PA1 B.sub.2 =lowest calibrated surface roughness PA1 Y.sub.1 =frequency intensity of B.sub.1 PA1 Y.sub.2 =frequency intensity of B.sub.2 PA1 Y.sub.3 =frequency intensity of the measured workpiece
The light projection technique is described by Shetty in "Laser Evaluation of Cutting Angle and Surface Finish in Scalpel Blades", Journal of Testing and Evaluation, 1982. The profile of a thin edge is projected onto an inclined screen, thereby producing a magnified shadow roughness profile image. The interference effect associated with a thick specimen and the surface are reported to give rise to spurious magnification and limit the specimen thickness. A roughness range from 2 to 4.7 microinches was found to be measurable.
An interferometry technique is described by Bennett in "Stylus Profiling Instrument for Measuring Statistical Properties of Smooth Optical Surfaces", Applied Optics 20, 1785-1802 (1981). He used two coherent beams split by a partially transmitting mirror to observe the fringe patterns in an interferogram.
The positional variation of reflected light is described by Shiraishi in "Dimensional and Surface Roughness Controls in a Turning Operation", Journal of Engineering for Industry, Vol. 112, p. 78-83 (1990). This measurement system uses double laser beams normal to the surface, and he discusses the variation of surface roughness and tries to link it to the signal variations from the photodiode.
The recent work of Marx and Vorburger, "Direct and Inverse Problems for Light Scattered by Rough Surfaces" reported in Applied Optics, Vol 29, No. 25 (1990) is of interest. The authors describe an experimental setup which they devised to perform non-contact measurement of surface roughness by comparing the specular and the scattered reflection components from a laser beam incident on the workpiece. Earlier researchers had employed a small number of photodetectors which were moved mechanically through a series of positions in order to observe the diffraction pattern over a relatively wide range of angular aspect of the light scattered from the workpiece. Marx and Vorburger improve on this approach by constructing a semicircular yoke which contains 87 photodiodes spaced 2.degree. apart about its periphery and which can be pivoted about the chord lying in its horizontal axis. The field of view of each diode subtends an angle of 1.5.degree., so that stepwise rotation of the yoke about its horizontal axis results in successively overlapped arcuate fields of view which can be combined into a hemispherical plot of the pattern of diffracted light intensity.
The characteristics of a laser beam which make it ideal for this application are the extremely low divergence, or angular spread, of the beam, and the sharply defined edges of the illuminated spot where the beam is incident on a surface. To understand the principle involved, one should imagine that a laser beam is directed onto an optically polished surface, and that a photodetector whose aperture matches the reflected spot size is positioned only a short distance away, where it can intercept the reflected beam. Over this short distance, the increase in beam diameter is miniscule, and, because of the high polish of the reflecting surface, the edges of the spot remain crisp. Therefore, one can assume that all of the energy is captured by the photodetector.
However, if the surface is not highly polished, then the reflection will be diffuse (a diffraction pattern); the spot edges will be blurry; and part of the energy will be spread over a wider angle. The coarser the finish, the more diffuse will be the diffraction pattern. In practice, diffraction patterns are observed which are not only relatable to the fineness of the surface finish, but also are characteristic of the illuminated object. Thus, a wire, a mesh screen, etc., yield patterns immediately recognizable as specific to these objects. When the object is a flat or smoothly curved surface, diffraction patterns result which are sufficiently repeatable to serve as identifiers of the finishing process.
However, commercial interest is not in the phenomena, but in the measurement of the surface finish. Although prior efforts in this area have suggested the potential for measuring surface finish through analysis of the diffraction pattern observed from a laser illuminated surface, prior instrumentation has not been practical for use in a manufacturing environment, either because of cost considerations or because of the equipment complexity. The equipment used by Marx and Vorburger, while intended as an experimental set-up, illustrates the complexity of prior equipment designed to capture and analyze such diffraction patterns.
It is an object of the present invention to provide a novel non-contact method for evaluating surface roughness of various engineering surfaces which reflect light and produce a diffraction pattern.
It is also an object to provide such a method which measures surface roughness on surfaces with variable geometry which would normally prohibit measurement by current instruments other than those for visual comparison.
Another object is to provide such a method which measures surface roughness of the workpiece without restriction to the orientation of the workpiece, i.e., whether normal or tangential or any other orientation to the lay.
Still another object is to provide such a method which provides quick, accurate measurements to reduce the processing time and the memory requirements of the computer for each workpiece.
A further object is to provide a method which allows the operator to interact in the form of a menu driven procedure.
A still further object is to provide novel apparatus for optical evaluation of surface roughness to provide accurate, convenient and quick multiple measurements of an engineering workpiece.